A diffuse background of X-rays is known to emanate from the
whole sky. The origin of this is not known although it is clear that at
least some of it comes from many distant active galactic nuclei which
are known to emit X-rays.
Below 1 keV the soft X-ray diffuse background is believed to be largely
galactic in origin. Thermal emission from hot gas is the likely source of
this flux, and for this reason its spectrum is dominated by emission
lines from highly ionized species.
In the X-ray band above 1 keV, the diffuse emission comes predominantly
from sources of extra galactic origin.
It is possible that all of the background is
due to discrete sources, but this hasn't been conclusively demonstrated.
Some truly diffuse emission of cosmic origin is still a possibility.
This would imply a vast amount of previously unseen matter in the